1. Field of the Invention
The present invention relates to an apparatus for purifying contaminated soil and to a method for remedying soil contaminated with pollutants.
2. Related Background Art
Chlorinated aliphatic hydrocarbon compounds such as trichloroethylene, and aromatic compounds such as phenol, are chemically very stable, and have been used in a large amount as raw materials for chemical or as cleaning solvents for semiconductors or machinery because of their high solubility for grease. On the other hand, their carcinogenicity has been pointed out. Thus organic chlorinated compounds leaked or discarded into the environment have become a source of wide range environmental pollution over the world.
Since well water, especially, is used in large quantities for industrial/agricultural use as well as drinking water, survey of the soil and groundwater contamination and a measure therefor are required urgently.
The number of the chemical substances under regulation is increasing year by year, and now is the time to make every effort to remedy the polluted environment worldwide.
To purify the soil and groundwater contaminated with organic compounds, there are two approaches: physicochemical approach and microbiological approach. For example, the physical/chemical approach includes heating or incinerating treatment of the dug-up contaminated soil, vacuum extraction of the pollutants from the contaminated soil, and aeration or adsorption treatment of the pumped-up contaminated groundwater. Neither of the vacuum extraction and pump-up aeration approaches, however, make the organic compounds harmless, requiring a further process to make them harmless.
On the other hand, the microbial remediation of the polluted environment (hereafter referred to as "bioremediation") has been attracting considerable attentions as a more eco-minded approach than physicochemical approaches, since it allows degradation of pollutants into essentially harmless substances. One embodiment of the bioremediation is in situ remediation treatment. This is to purify the contaminated soil by directly introducing a microorganism capable of degrading pollutants into the polluted environment (soil, groundwater etc.), by propagating the native microorganisms capable of degrading the pollutant in the polluted environment, or by directly introducing certain substances to elicit or improve the pollutant-degrading activity of the native microorganisms.
For example, DE 4003362 A1 discloses a purification method in which the contaminated soil is mechanically agitated and crushed to uniformly mix with a degrading microorganism and nutrients.
EP 0620055 A1 describes a method in which the contaminated soil is mechanically agitated while injecting compressed air and the exhausted gaseous pollutant is collected for purification.
DE4028055 A1 describes a purification method in which the contaminated soil is dug while sprinkling microorganisms to the soil.
Alternatively, there is an approach to move the pollutant such as organic chlorinated compounds in the environment. U.S. Pat. No. 5347070, for example, describes a method in which a plurality of electrodes are inserted into the soil polluted with organic chlorinated compounds and sending an electric current to the soil to raise the soil temperature, so that the pollutants are vaporized to be removed. Further, U.S. Pat. No. 5340570 discloses a method in which pollutants are removed by electroosmosis due to an electric field applied to the soil.
Japanese Patent Application Laid-Open No. 7-112177 discloses an apparatus for effective in situ remediation of polluted soil, which comprises soil-agitating means for agitating the soil while penetrating it, means for supplying the agitated soil with a dispersion of a microorganism, and means for supplying a material to activate the microorganism. Disclosed also is a method for remedying a contaminated soil by using this apparatus, which comprises steps of: boring and agitating the contaminated soil using a device capable of boring and agitating the soil, feeding the soil with a dispersion of a microorganism capable of treating the contaminated soil, and agitating the soil for a prescribed period of time while injecting a substance(s) to activate the microorganism. These steps as one cycle are repeated in different places according to the area and site of the contaminated soil. As described above, various types of in situ bioremediation have been proposed.
The present inventors have learned from the previous studies that it should be avoided as much as possible for the introduced foreign microorganism or compounds which do not originally exist in the soil but required for in situ bioremediation to affect the ecological system of the polluted environment, particularly after the remediation.